It is often desirable to measure the angular position of a rotatable shaft with a high level of precision—to a fraction of a degree, for example, and many conventional resolvers and encoders can provide this level of precision. In general, such devices either include a) semiconductor elements, found in Hall effect sensors, for example, or b) optical elements such as lasers or lenses or glass elements. Such elements will not function and/or will not last long in certain environments. For example, certain high radiation environments can damage semiconductor elements as well as the optical elements in an optical encoder system.
A high-radiation environment in which precise angular measurements are required is the region of the rotating shaft portion of a neutron chopper. Neutron choppers are mechanical devices that include a rotating mass of neutron-blocking or absorbing material, with one or more through openings. The mass of neutron-blocking or absorbing material is sufficient to substantially block a beam of neutrons, and the rotational speed of the mass (or the shaft supporting the mass) is controllable so that the openings are aligned with the beam of neutrons at precisely determined times in order to let through carefully calibrated bursts of neutrons. Often, the rotation of the shaft that supports the rotating mass must be synchronized with a clock or other timing signal. The timing signal, for example, might control both the production of neutrons and the movement of the neutron chopper and/or other operations in a facility requiring controllable bursts of neutrons.
Many conventional resolvers and encoders are not well-suited for use in the environment in which a neutron chopper operates. In addition to the radiation in the form of the neutron beam, the high rotation speed of the chopper (30,000 to 40,000 RPM, for example) can damage delicate sensor components. These elements must therefore be provided with adequate shielding or be replaced when they wear out prematurely.
A variable reluctance sensor is a good candidate for measuring shaft position in such environments. Variable reluctance sensors generally include a permanent magnet sensor core around which a conductive wire is wrapped. Changes in the magnetic flux produced by the sensor core affect a current flow in the wire wrapped around the magnet. The path of the magnetic flux may be affected by moving a body of ferromagnetic material toward and away from the sensor core. Placing a body of ferromagnetic material near the magnet concentrates the flux path in the body of material because the body exhibits a lower reluctance than the surrounding air. Moving the body of ferromagnetic material away from the sensor core increases the reluctance of the flux path and also affects a current flowing in the wire that surrounds the sensor. Alternatively, the sensor could be constructed of a non-magnetized ferromagnetic core, also with a conductive wire wrapped about it, rather than a permanent magnet core. In this alternative, an electrical current is passed through the coil, and is likewise affected by relative motion away or towards the sensor of an adjacent ferromagnetic body.
The body of ferromagnetic material that moves toward and away from the sensor core may be a wheel or gear having teeth. As the wheel or gear rotates, teeth approach, move past, and move away from the magnetic sensor. This causes the magnetic flux to vary in a regular manner, and this variation produces a regular signal in the wire surrounding the permanent magnet. This signal is provided to a controller that, by counting peaks in the signal, for example, can be used as part of a resolver to measure the movement of the gear and thus the shaft to which it is attached. Knowing the number of teeth on the gear allows the speed and position of the gear to be determined by conventional measurement circuits. The portion of such sensors that is exposed to radiation and/or the harsh environments found in neutron choppers constitutes merely ferromagnetic materials and wire. These components are not adversely affected by radiation and thus are good candidates for angular position sensors. It would therefore be desirable to provide an improved angular position detector based on variable reluctance sensors that is adapted for use in a high radiation environment and that can provide sub-degree measurement accuracy.